Subatomic particles such as protons or neutrons, have the quantum property of spin. The overall spin of an atomic nucleus is dependant on the number of protons and neutrons in the isotope. Atomic nuclei with odd atomic numbers, such as hydrogen (1H), have a net non-zero spin and therefore have a magnetic moment, while molecules with an even atomic number, such as Carbon-12 (12C) have no net spin. When placed in an external magnetic field, polar molecules “precess” around an axis in the direction of the field. Subatomic particles align in either of two energy states: a low energy state or a high energy state. An external oscillating field will result in resonant aborption of energy by the subatomic particles. Resonant frequencies for particular molecules are directly proportional to the strength of the applied magnetic field, and are also related to the chemical composition of the molecule.
The property of magnetic spin is used in Nuclear Magnetic Resonance Spectroscopy and Magnetic Resonance Imaging to image target materials such as biological specimens, including human anatomic structures. In current, conventional Magnetic Resonance Imaging, a net polarization is developed by inducing a strong magnetic field. As noted above, subatomic particles within the nucleus of the atom align in either of two energy states, a high energy state or a low energy state. The alignment of the subatomic particles in these two energy states is responsible for the magnetic properties of target areas detected by magnetic resonance imaging and used to develop the image. The net magnetization vector has two components—a longitudinal component and a transverse component. The longitudinal component of the vector is related to an excess of aligned molecules (typically in biological molecules hydrogen atoms or protons) in the low energy state. The transverse component is due to the formation of coherence between the two subatomic energy states. Obtaining a magnetic resonance image includes two elements: an external magnetic field and a series of radio frequency pulses. In the case of biological specimens such as human anatomy, a radio frequency pulse of an appropriate energy level to excite protons within the specimen is applied to the specimen in the presence of an external magnetic field that ensures that only protons in a particular plane within the target are “on-resonance” and contribute to the signal. The radio wave disrupts processing molecules within the specimen. Recovery of longitudinal magnetization within the magnetic field after the radio pulse is discontinued is called the T1 relaxation of the specimen. Loss of phase coherence in the transverse plane is called the T2 relaxation. Both T1 and T2 relaxation result in electromagnetic waves in radio frequency wavelengths that can be detected by a receiving coil. Other properties of the signal can be detected using a variety of techniques with the external field, including using additional gradient magnetic fields, sending coils, and pick up coils to add information to the MRI image.
Conventional MRI has revolutionized medical care by providing crisp and useful internal images of human anatomy. However, conventional MRI has a number of disadvantages in the current state of the art.                1) First, conventional MRI can only image one slice of an anatomic specimen at a given time. Conventional MRI uses sending coils and receiving coils to generate repeated radio pulses through the specimen, with multiple signals read at one or a limited number of receiving coils. Multiple repeated signals with progressive signal averaging is needed to develop the image in each slice. Conventional MRI therefore requires significant time to obtain a useful image. The long time required to obtain an MRI image results in limitations in patient utility. For example, children, elderly adults, individuals with cognitive problems, and many other patient populations have difficulty staying within a conventional MRI magnet for the period of time needed to obtain optimal images.        2) Secondly, conventional MRIs require development of extremely large magnetic fields. Significant separation is required between MRI facilities and other facilities because of the significant magnetic effects on metal objects (such as surgical objects) and electronic equipment. The requirement of utilization of large magnetic fields prevents many patients, including individuals with metal prosthetic devices, individuals with shrapnel or bullet wounds, or individuals with pacemakers and deep brain stimulators, from obtaining MRIs.        3) Third, large, superconducting magnets are required to develop magnetic resonance images. Conventional MRIs are therefore bulky, and require dedicated rooms or suites. In some cases, large relatively lower quality mobile MRI devices are available that are transportable in trucks, however portability even in these devices is limited.        4) Finally, as can be understood from the above, conventional MRIs require a significant capital expense. The expense associated with installing and maintaining an MRI facility limits the use of this technology to typically hospital environments in first world countries.        
The significant disadvantages of conventional MRI limit utility of that conventional technology. There is a need for a more portable, less cumbersome, and less expensive MRI scanning technology.
Recently, Clarke et. al. disclosed utilizing conventional SQUID based systems for detection of Nuclear Magnetic Spins (Clark et. al., U.S. Pat. No. 7,187,169). While disclosing a system that can detect low field NMR, and can be used for Magnetic Resonance Imaging, the system described by Clark et. al. is based on SQUIDs, which require cryogenic cooling. Further, SQUID systems are often bulky, expensive systems with limited capacity to allow for patient mobility.
Below, we describe a new atomic-magnetometer based, magnetic imaging technology, method and device. This subject technology can be more economical, portable and wearable. The technology we describe incorporates sensitive magnetic detectors that can function using significantly lower magnetic fields to develop signals. Our device is expected to result in lower cost to the consumer, and lower power consumption, potentially allowing the development of a device that can be used in an office practice, or more effectively in less advantaged countries. Further, our device is expected to use a lower magnetic field than conventional MRI scanners, opening the possibility of inter-operative magnetic imaging as well as imaging within an ambulance, on a commercial airliner or helicopter in the case of airborne emergencies or in a typical physician office environment.
This atomic-magnetometer based magnetic imaging system may be able to rapidly image the brain using an array of magnetic sensors.
The atomic-magnetometer based magnetic imaging system addresses sensor cross talk, shielding, mobility, and necessary algorithms for sensor calibration and data interpretation to allow development of a working magnetic imager using a new sensor type that reduces size and expense and increases portability.
For example, we now disclose, first that atomic magnetometers have several characteristics that may require new methodologies to deal with the issue of sensor cross-talk. Atomic magnetometers generate a magnetic field in the course of operation. This particular characteristic of these sensors may require special methodologies of operation and signal processing in order to produce a system that can actually measure MRI-like magnetic fields.
Secondly, many atomic magnetometers may detect magnetic fields along only very specific vectors. While potentially advantageous for source localization, this added directional capability requires specific data processing and design elements to allow development of a useful array for measuring MRI signals.
Third, the compact size of some atomic magnetometers, allows the development of an entirely new type of device and method. Specifically a mobile device and method allows the subject to freely move the head, neck, and in some cases allows ambulation while being continuously monitored. In some embodiments wearable magnetic imaging devices may be developed including wearable caps, helmets, blankets, or clothing like articles such as vests or wraps for arms, legs or torsos. A general term, “apparel,” will be used in this description and in the claims to refer to items that are wearable while retaining general mobility. Apparel can include such caps, helmets, blankets or clothing described above, and also draperies, garments, wraps, casts, frames, structures, outerwear, gloves, shoes, masks, covers, suits, equipment or other items that can be or are designed to be worn by a human or other biological specimens while remaining relatively mobile. We use the phrase “specimen” to refer to portions of a person, a whole person, biological samples, or nonbiological samples. Further, in some embodiments, sheets of sensors may be used for imaging of biological or non-biological samples.
Fourth, some sensitive atomic magnetometers, such as SERF magnetometers, require provisions for magnetic shielding or field cancellation to be clinically useful. We enumerate a variety of devices and methods for these purposes.
Fifth, the possibility of utilizing large numbers of sensors allows the development of active sensor selection methods, which can in real time detect the optimal number and location of active sensors for a given clinical session or application.
Sixth, the atomic magnetometers operate at elevated temperature and thus may require some level of thermal isolation from the patient. This can come in the form of an insulating layer or active cooling.
Seventh, large numbers of atomic magnetometers can be arrayed around a body part of a subject or target specimen. Therefore, instead of developing information from the brain slice by slice using time average of multiple signals, induction of one or more RF signals may be utilized to generate a field in the specimen and precession, with multiple sensors capturing relaxation information from the entire specimen. This process will in some embodiments require much less time to obtain a useful image.
Eighth, the high sensitivity of some atomic magnetometers can require a smaller magnetic field, allowing in some cases a smaller magnetic generator and allowing the development of a portable, wearable imaging device. Therefore, atomic magnetometer based systems may allow the development of a low field MRI.
In this application, we describe various devices and methods that, in particular embodiments, serve the relevant clinical needs of mobility, ease of use, and lower cost. In various embodiments, atomic magnetometers, arrayed around a patient's head in a mobile helmet, provide the significant advantages of allowing more comfortable monitoring which is advantageous in many cases, particularly, for example in optimally measuring children and disabled patients. We describe a number of clinical utilizations made possible by a portable array. For example, we describe that the array proposed can, in addition to measuring NMR signals for the purposes of MRI, can also measure biologically generated signals. We also describe the components of a portable array in relation to signal processing, inter-sensor interference, and magnetic shielding.